Transmission electron microscopy (TEM) study on bornite and impactite.

摘要

Because of the problem of sample heterogeneity, research on bornite and impactite has been difficult with conventional bulk sample methods, such as x-ray diffraction, due to their low spatial resolution. However, with its capacity to provide imaging, diffraction, and chemical information from very small domains in samples, transmission electron microscopy (TEM) was intensively applied to study bornite and impactite in this thesis work. There are two parts in this thesis that describe the studies on bornite and impactite respectively. The work on bornite consists of three chapters, and each one aims at solving a particular problem. In the first chapter, two atomic superstructure models (4a-1 and 6a-1) for bornite have been determined from high-resolution TEM images; these are the first three-dimensional structure models of bornite superstructures. These models indicate that non-space-group extinctions (anomalous diffraction) can arise purely from metal/vacancy ordering. It also has been found that bornite samples are generally not homogenous, and the most frequently observed phases are the 2a and 4a superstructures. The second chapter describes the investigation of Fe/Cu ordering in the superstructures, using electron energy loss near edge structure (ELNES) and quantum mechanics first principles (density functional theory (DFT)) methods. The results indicate that metallic bonds form between the nearest-neighbor metal atoms and covalent bonds form between nearest-neighbor sulfur and metal atoms in bornite. Thus, ionic bonding concepts are not suitable for discussing the properties of bornite. Also in this chapter, it is shown that vacancies are probably associated with Fe atoms, and Fe/Cu therefore order in the superstructures, based on the comparison of experimental and theoretically calculated ELNES spectra. In the last chapter on bornite, we have proposed a new 2a Fe/Cu ordered structure model that is based on DFT calculations.; Research on impactite from Henbury, Australia, is described in the second part of the thesis. Through the application of energy-dispersive X-ray spectroscopy (EDS), convergent-beam electron diffraction (CBED), and selected-area electron diffraction (SAED), chemical compositions and space groups have been determined for all sub-micrometer crystalline phases observed in the impactite, including fayalite, magnetite, α-quartz, hercynite, diamond, and a Ni-sulfide phase, as well as abundant silica-rich glass. The origin and evolution of the crystalline phases are discussed in a thermodynamic context, and the temperature and pressure for formation of the impactite have also been estimated.